NATURAL HISTORY

Both elkhorn and staghorn corals are named for their many large branches, which look similar to elk or deer antlers.

DESCRIPTION: Corals are actually colonies of many tiny invertebrate animals called polyps, which together form a hard calcium carbonate skeleton. Acroporids are reef-building types of coral with large branches ranging from a few inches to more than six feet in length. They vary in color from yellow to purplish brown.

HABITAT: Acroporids live in shallow saltwater seas at depths ranging from zero to 98 feet. They are typically found in water temperatures from
66 to 80 degrees Fahrenheit.

RANGE: Elkhorn and staghorn corals are found throughout the Florida Keys, the Bahamas, and the Caribbean islands, including the U.S. Virgin Islands and Puerto Rico.

BREEDING: The dominant mode of reproduction for acroporids is asexual fragmentation, in which branches break off and reattach to the substrate to form new colonies. Individual colonies are hermaphrodites, having both male and female reproductive organs in a single individual. Once a year, in late summer, sexual reproduction occurs via the release of millions of reproductive cells into the water.

LIFE CYCLE: Polyps and the reefs they form can live for hundreds of years.

FEEDING: Acroporids host symbiotic algae called zooxanthellae. While corals get most of their nutrients from the byproducts of the algae's photosynthesis, they also have barbed, venomous tentacles they can stick out to grab zooplankton and even small fish.

THREATS: Global warming is the overarching threat facing Caribbean corals today. Increased water temperatures cause mortality and bleaching, a process in which corals expel their symbiotic algae due to stress factors. The survival of corals is also threatened by ocean acidification, which impairs the ability of corals to build their hard skeletons; also, unabated, acidification could eventually begin to erode coral reefs. Land-based sources of pollution increase nutrients and allow large, fleshy algae to proliferate and overgrow corals. Agricultural runoff reduces oxygen levels and introduces pathogens. Overfishing and disease have caused a reduction in a number of important predatory fishes (such as groupers) and herbivores (such as parrotfish). Reductions of these fishes leads to an increase in organisms preying on acroporids, including the short coral snail, fireworm, and damselfish.

POPULATION TREND: Since 1970, Caribbean coral populations have collapsed throughout their range, declining by up to 97 percent. Coral cover has been reduced and space has been opened up on reefs at previously unheard-of speeds. In just 30 years, 80 percent of these corals disappeared from Caribbean waters. A mass-bleaching event in 2005 killed 20 percent of the remaining coral. This die-off starkly contrasts with the corals' ability to persist through the Pleistocene and Holocene mass-extinction periods.